In an image forming apparatus, such as a copy machine, a printer, and a compound machine, based on image data which is read by a document reading device, a toner image is formed on an image carrier provided in the image forming apparatus, and after the toner image is transferred onto a recording sheet supplied from a tray, the toner image is fixed, whereby permanent image formation (or printing) is performed.
Since various types of sheets are used for printing, a coated sheet is characterized in that its air permeability is relatively low due to its coated surface, further, under a high humidity environment, moisture tends to be trapped between adjacent stacked coated sheets. Accordingly, adhesion force tends to be high between the stacked sheets, and a top coated sheet is not easily separated from a second stacked coated sheet on the tray, whereby the coated sheets are not easily supplied to the apparatus, resulting in erratic sheet feeding. Still further, when the recording sheet is conveyed under said high humidity, and printing is conducted on the sheet, paper sheet debris generated from the recording sheets accumulates in the conveyance path, interfering with proper operation of optical sensors mounted in the conveyance path to detect the recording sheets. To overcome these problems, an image forming apparatus is proposed in which a dehumidifying heater is mounted within each tray to decrease the humidity of the coated sheets.
When plural jobs are executed based on a predetermined procedure on the image forming apparatus, these jobs are required to be effectively executed. After a switch is turned on, or during an electrical power saving condition in which temperature of a fixing device is controlled to be lower, it takes a long time until the temperature of the fixing device reaches a printable condition so that waiting time is essential while no printing is executed. Since a tray to supply the sheets or a special sheet type of printing sheet is instructed for the specific job, some sheet types require low fixing temperature. In such case, a job to be operated by low fixing temperature is firstly conducted, so that the waiting time, which starts immediately after the switch is turned on, can be reduced whereby the printing operation is more effectively conducted, which is disclosed in the Unexamined Japanese Patent Application Publication No. 2003-280, 461.
However when humidity control is conducted in the paper tray as described above, if a tray incorporating a dehumidifying heater is used, a relatively long time is necessary from the activation of the image forming apparatus or from the start of dehumidification to the completion of the dehumidification. In the conventional art, the image forming apparatus conducts various jobs in a sequential order of the entrance of the printing instructions. When the image forming apparatus forms the images on the coated sheets which tend to be influenced by humidity, time for dehumidification is required for the coated sheet, so that waiting time becomes long, which results in undesirable printing efficiency. The image forming apparatus of the above patent document tries to increase the productivity rate, while using the difference of fixing temperature due to the sheet type, but it does not overcome the undesirable waiting time during the dehumidification of the sheets, which causes a major problem.